1. Field of the Invention
The present invention relates to sheet stacking devices and image forming apparatuses.
2. Description of the Related Art
A known example of a sheet stacking device is a device which stacks sheets that are ejected after images are formed thereon.
In order to stack a larger number of sheets on a stacker tray of a sheet stacking device, it is necessary for the stacker tray to have a large driving range in the vertical direction. For this reason, the driving range within which the stacker tray moves must not have any obstacles and must therefore always provide an empty space for the stacker tray to move therein.
However, users that desire to utilize small spaces efficiently tend to consider the space below the stacker tray to be a dead space. In consequence, this space below the stacker tray is commonly used as a space for placing supplies such as sheets and toners for an image forming apparatus or as a space for placing office supplies such as trash bins.
As sheets are sequentially stacked onto the stacker tray to cause the stacker tray to descend, an object such as a stack of sheets placed below the stacker tray in the above-described manner become an obstacle and unfavorably prevents the stacker tray from descending further. This causes an error or failure in the apparatus. Therefore, when stacking a large number of sheets, the user needs to check whether there is an object placed below the stacker tray.
Japanese Patent Laid-Open No. 8-26570 discloses a technology in which an optical sensor is used to detect whether or not there is an object acting as an obstacle placed below the stacker tray.
FIG. 15 illustrates a sheet stacking device of related art that is equipped with such an optical sensor. A sheet stacking device C is provided in a sheet processing apparatus B and includes a catch tray 12, serving as an example of a sheet stacking portion, which is movable vertically so that ejected sheets from an image-forming-apparatus main body A can be stacked on the tray 12. A reflective optical sensor 1101 is provided at a lower section of the catch tray 12 and is configured to detect an object 1102.
Japanese Patent Laid-Open No. 2001-348162 discloses a technology in which a switch is used to detect whether or not there is an object acting as an obstacle placed below the stacker tray.
FIG. 16 illustrates another sheet stacking device of related art that is equipped with such a switch. This sheet stacking device includes a stacker tray 13 serving as an example of a sheet stacking portion that is vertically movable in accordance with the number of ejected and stacked sheets. The stacker tray 13 includes a stationary member 2 fixed to the undersurface of the stacker tray 13 and a damper member 3 set rotatably with respect to the stationary member 2 and biased in a direction away from the stationary member 2.
The stationary member 2 is equipped with a switch 41 that detects whether the damper member 3 is within a predetermined distance. In FIG. 16, reference numeral 1102 denotes an object acting as an obstacle placed below the stacker tray 13.
As the number of stacked sheets increases to cause the stacker tray 13 to descend, the damper member 3 eventually comes into contact with the object 1102. In this case, if the stacker tray 13 descends even further, a projection 31 provided on the damper member 3 fits into a recess in the switch 41 so as to turn on the switch 41, whereby it can be detected that the object 1102 is below the stacker tray 13.
However, the sheet stacking devices and image forming apparatuses of the related art described above may have problems. For example, with a sheet stacking device that is configured to detect an object with a reflective optical sensor as shown in FIG. 15, the detectability of an object is affected by the reflectivity or the color of the object or by the operating environment. Furthermore, since a reflective optical sensor has a limited detection range, multiple optical sensors are required in order to detect the entire area below the catch tray. This can lead to complicated object-detection control in addition to an increase in power consumption and cost.
On the other hand, in a sheet stacking device that is configured to detect an object with a switch 41 as shown in FIG. 16, an object can be detected using a single switch 41. However, with the configuration in which an object is detectable with the switch 41 shown in FIG. 16, the stacker tray 13 that moves vertically relative to the apparatus main body has the object-detecting damper member attached thereto in a rotatable fashion, causing the vertically movable stacker tray 13 to have a complex structure. The reason this complex structure is inevitable is that, in order to make the entire area below the stacker tray 13 into a detectable range, the damper member needs to be rotatably attached to the vertically movable stacker tray 13 so as to cover the entire undersurface of the stacker tray 13.
Moreover, with the configuration in which an object is detectable with the switch 41 shown in FIG. 16, the presence of an object can be detected only after the stacker tray 13 (i.e., the damper member 3) comes into contact with the object. Therefore, when the switch 41 detects the object 1102, even though the stacker tray 13 can be stopped from descending further, it is difficult to stop the operation of the image-forming-apparatus main body immediately, thus undesirably causing the image forming operation to continue. There may also be a case where pre-stacked sheets still remain inside the sheet stacking device. Therefore, after stopping the stacker tray 13, the sheets remaining inside the image-forming-apparatus main body and the sheet stacking device must all be ejected onto the stacker tray 13.
However, when ejecting and stacking these sheets onto the stacker tray 13, the usual sheet stacking control operation where the stacker tray 13 is lowered in accordance with the ejection of sheets cannot be performed, thus resulting in reduced stackability of sheets on the stacker tray 13.